Leguminous plants fix nitrogen from the air and convert it to organic nitrogenous compounds used by the plant for protein synthesis. Nitrogen fixation in leguminous plants is possible because of the symbiotic relationship with bacteria of the genus Rhizobium and Bradyrhizobium, which forms nodules on the roots of legumes. Different species of rhizobia cause nodulation in specific legume genera. Maximum symbiotic nitrogen fixation occurs when plant and bacteria are properly matched, and when nodule formation is maximized. Bradyrhizobium japonicum is associated with (nodulates) soybeans, Rhizobium leguminosarum biovar trifolii with clovers, R. meliloti with alfalfa and sweetclovers, R. leguminosarum biovar viceae with peas and vetches, and R. leguminosarum biovar phaseoli with garden variety beans.
It is common practice to inoculate leguminous plants with rhizobia to aid nodule formation. Inoculation can be accomplished by pre-inoculating seeds, or either inoculating seeds, or placing inoculant in-furrow at planting time. Previous methods of producing an inoculant have included mixing an active, living rhizobial culture with a carrier such as humus or peat. The moist carrier maintains the bacteria in a living state.
An early method of preparing inoculants was by converting the bacteria to a dormant state. U.S. Pat. No. 3,168,796 to Scott, et al describes a method of preparing an inoculant including a step of freeze-drying. This process must be done rapidly to prevent cell rupture. The dried, ground bacteria are mixed with a powdered carrier such as kaolin or montmorillonite. Freeze-drying gives a high initial recovery of bacteria, but the inoculant does not remain stable for long storage periods.
Another method of preparing a dry, dormant inoculant is cited in PCT Published Application No. 92/08355, published May 29, 1992. The described process produces a dry, dormant bacterial composition wherein the water content is less than 5% by weight and at least 10.sup.9 viable bacteria per gram of the composition. The carrier is a clay mixture of montmorillonite and kaolinite which has an essentially neutral pH. Such a dry, dormant bacterial composition is available commercially under the trademark Nitragin Gold.
Biocidal compositions containing bacteria or fungi which combat insects, fungi or the like may also be prepared using the slow drying process described in the foregoing European patent publication. Interest in dry, dormant bacterial products has increased due to recent interest in biological pesticides as an ecological alternative to conventional pesticides.
Seed coating is a popular method for applying bacterial inoculants and other beneficial bacteria such as biopesticides to the target plants, particularly for alfalfa seeds. Several methods may be used to make coated alfalfa seeds, including dusting, pelleting, and film coating. Dusting of alfalfa seeds with a dry, dormant inoculant containing R. meliloti gives rise to a product which has an excellent shelf life, but the dust is not completely adhered to the seed, resulting in release of loose dust to the surrounding atmosphere whenever the coated seeds are handled in the open.
Film coated seeds have the advantage of not releasing dust. However, known film coating processes incorporate water in the process. Bacteria from a moist product, or which are rehydrated during the coating process, have poor viability upon subsequent drying.
A need persists for a method of preparing a coated seed wherein the bacteria in the coating have been pre-conditioned for survival after temporary rehydration. The adverse effects of temporary rehydration on previously dried, dormant bacteria are well known.